"Just by my being here, speaking to all of you -- me -- that alone," he said, "is unexpected."

Scott, as he explained in his speech, usually doesn't talk. It's not that he can't; he has the ability and the vocabulary. But he also has autism and so often chooses not to speak, outside a few trusted peers and teachers. People with autism spectrum disorder often have problems with social, emotional and communication skills.

Plano Senior High School Assistant Principal Bryan Spiritus, who has known Scott for two years, said he's heard the Texas student speak daily, though the 17-year-old nearly always was just thinking aloud to himself.

"I was concerned that standing in front of his 1,400 peers and the 8,000 people in the arena, that he might freeze up," Spiritus told CNN. "But about a minute into it, I realized that wasn't going to happen."

Scott's mother and brother, a brain tumor survivor who has given speeches for children's charities, helped him write the speech, CNN affiliate KTVT reported. Scott also worked closely with faculty member Brittney Love, who was there to help him if he froze, changed his mind or couldn't finish.

"I thought he would do great," Love told CNN. "He doesn't usually get nervous."

In the year that she's worked with Scott, Love said he began answering questions, telling others about his weekend and even showing physical affection.

And Scott was chosen as commencement speaker through a voluntary audition process.

Scott's speech was "probably one of the best graduation messages I've heard," Spiritus said, adding that Scott wasn't chosen simply because of his disability but also based on presentation, message and the ability to motivate his peers.

His six-minute oration was met with laughs, cheers and a standing ovation from the audience. Love cried.

"I was so overwhelmed. I was literally so proud," she said. "I was just so happy to be a part of that."

Scott closed his speech with this: "Remember, if you are following in someone else's footsteps, you will only get where they want to go. Be the unexpected, like me. Do the unexpected for the benefit of others. Live the unexpected for your own happiness."